Carbenes can be produced and reacted safely and rapidly using a continuous-flow process rather than batch processing, say scientists who exploited IR spectroscopy to characterise the reactions.

The new process was illustrated in Chemistry - A European Journal by researchers in Cardiff, UK and Gaillac, France, using the reaction of methyl phenylacetate with p-acetamidobenzenesulfonyl azide to produce diazo methyl phenylacetate. The diazo compound was reacted in situ without prior isolation with substrates such as 3-bromoaniline in an N-H insertion reaction.

The reaction temperature and the residence time of the reactants in the flow system have similar effects to each other, so the latter can be reduced by increasing the former. However, safety from the explosive diazo intermediates at higher temperatures is assured by incorporating an inline liquid-liquid extraction step with a membrane-based system.

Inline IR spectroscopy allowed the research team to study the kinetics of the reactions and to follow the course of the reactions. Other reactions that were realised with this scheme were O-H and S-H insertion reactions and cyclopropanation and the starting compounds could be alcohols, cyclopropanes, thiols, carbamates and amines.

The process was also scaled up to allow the preparation of gram amounts of the products.